Issue #114

Jul/Aug 2016

I don't know how many unicorns PSI Audio had to milk to fill up this 20'' tall box full of magic. Actually, it's not really a milk box; it's more like a wedge of Swiss cheese. Coincidentally, its multifaceted shape reminds me of the polyhedral dice used in role-playing games about dungeons and magical beings. Regardless, the shape has been optimized for placement in a corner, where the AVAA C20 can be most effective in reducing unwanted room resonances. Inside the unit is a pressure-gradient mic mounted behind a micro- perforated screen carefully tuned to offer a specific acoustic impedance boundary. Also inside is an acoustic-suspension driver, which is mounted behind its own speaker grille—like impedance boundary. An analog amplification circuit propels the driver with a velocity that's in opposition of the pressure measured by the screened mic, in order to "zero" the acoustic pressure at the wall boundary or corner junction where the device is placed. Hence the name — Active Velocity Acoustic Absorber. To put it more simply, the AVAA is an active bass trap. Because it works with pressure, it makes no sound of its own. In fact, it's the most impressive audio product that I've seen and not heard.

First of all, what's the purpose of bass trapping, and why do you want to absorb bass when most people want to hear more bass? Well, here's the conundrum. Within enclosed spaces — especially smaller rooms with solidly built walls — as acoustic energy is reflected off of the room surfaces, some of that energy is strengthened while some of that energy is cancelled out. The frequencies of sound that are most affected have half-wavelengths that are low-order divisors of the axial dimensions of the room, resulting in standing waves at specific bass frequencies. Meanwhile, the source locations of the sounds in the room — as well as the location of your ears — determine whether you experience augmentation or cancellation of those modal frequencies. Therefore, you'll hear too much bass energy at certain frequencies and room locations, but at other frequencies and locations, you'll hear less bass. Importantly, the bass energy that you do hear will have a time component to it, because certain frequencies will resonate longer than others, taking time to settle down. If left untreated, these room modes and resonances will not only reduce the low-frequency accuracy of what you're hearing in the room, but excessive settling time of these resonances can mask detail at all frequencies.

If your inclination is to reach for an EQ as a quick fix, keep in mind that EQs can only boost or cut in frequency domain, with only a secondary effect in time domain. In other words, the ringing would still be there if EQ'ed, only altered in amplitude. Needless to say, rooms should be treated in a balanced manner with both broadband absorption and bass-trapping to mitigate standing waves and normalize decay time across all frequencies. Unfortunately, traditional bass traps take up a lot of surface area and a lot of volume, due to the material thicknesses (or chamber sizes) required to be effective at greater wavelengths. Also, many commercially available bass traps are better at absorbing the mids and the highs than they are the lows, so too much bass trapping, even if the bass frequencies have yet to be fully tamed, can result in a room sounding excessively dead. That's where the AVAA comes in.

If you've already treated your room with broadband absorption, but you've reached a limit to how much bass absorption you can add — due to space restrictions, fear of over-treating higher frequencies, or even for aesthetic reasons — the AVAA C20 should be your next step. This active bass trap is able to counteract room resonances by creating "anti-resonances" from 15—120 Hz, and its effectiveness at reducing standing waves at those frequencies is equivalent to busting a hole in the wall that's 5-20× the size of the AVAA. I purchased two AVAA C20 units, and after some experimentation with room placement, I settled on two opposite corners of the room. Although a sensitivity control is on the back of the AVAA, it's recommended you start with the trim pot on the CAL setting, unless your room is overly reactive (or smaller than 110 sq ft in floorplan). Otherwise, there's nothing left to do after you move the AVAA into place, plug it into an AC outlet, and power it on. The difference in low-frequency accuracy between having the two AVAAs powered on in my room, versus turned off, is not at all subtle — it's immediate and obvious. I was in the middle of a three-day, attended mix session with the band E when I installed the AVAAs. I've mentioned in the past that E's bass-heavy sound is a challenge to record and mix. On the second morning, the band came in after everyone had listened to the first day's mixes at home. The consensus was that there was no consensus — no one could agree on the relative levels of the bass-focused instruments in the mix. That's when I powered up the AVAAs, and all of the band members, even though they were sitting or standing in various spots throughout the room, heard the bass energy in the room clear up.

Jason Sanford's home-made instruments and amps, as well as Gavin McCarthy's hole-less kick drum, had contributed a lot of low-frequency energy to the recordings at very specific bass frequencies. Before the AVAAs were set up, some of that energy was overwhelmingly loud in the mix when heard from certain points in the room — but severely underrepresented when heard from other points. Meanwhile, the sustain of that energy was exaggerated enough that it was difficult to hear the intricacies in the band's performance, as if a dark fog was obscuring the view of everything else in the mix. In short, there was not enough "BOO..." in the bass — but way too much "...OOOMMMmmmnnn" clouding the overall image. Once we turned on the AVAAs, we could all hear the "BOOM" clearly — and everything else in the mix too. Needless to say, we abandoned all of the first day's mixes and started over — sans dark fog. Importantly, even though we had lost a whole day's worth of work, we still ended up finishing on time two days later, because we were able to make better informed decisions and work more efficiently due to the much clearer picture made possible by the AVAAs.

Being the gear geek, I pulled out my Cross·Spectrum—calibrated Dayton Audio EMM-6 mic [Tape Op #96], fired up my ADAM S3-A monitors [#66] and Sub12 subwoofer [#69], and took measurements in various spots throughout the room. If you were at all skeptical of my subjective statements above, please take a look at my before/after waterfall plots of the measured room response at mix position goo.gl/hBZXXT. As you can see, the two AVAA C20 traps are very effective at taming the low-frequency resonances in my room, even down to 28 Hz. With the AVAAs turned on, peaked ridges in the waterfall plot are cut back, and furrowed valleys are filled in, and the resulting spectral decay is significantly smoother and more evenly damped.

I should also note here that I have twelve of the legacy wood-panel RealTraps diaphragmatic bass traps [Tape Op #36] and nine of the current RealTraps models equipped with limp-mass membranes [#38, #48, #85] mounted on the walls and in the corners of my control room, as well as a Helmholtz resonator built into the platform below the couch at the rear wall (along with mid/high—band absorbers and diffusors strategically located). The room was already well treated, but as the plots make clear, there was still an opportunity for improvement, and the AVAAs have excelled in that regard. Each AVAA C20 is a $2000 investment. For that kind of money, you can purchase half a dozen or more premade bass traps. But as I mentioned above, if you're already reaching the limits of treating your room with passive absorbers, an AVAA or two should be your next consideration. Moreover, I've learned through research and personal experience that a roomful of premade traps isn't nearly as effective at frequencies below 60 Hz as even a single AVAA. Plus, an AVAA is a lot smaller and easier to take with you. If you factor in the confidence you'll gain and the time you'll save while recording and mixing in a more accurate-sounding room — and the improvements you'll see across all of your work — the AVAA C20 becomes a smart investment.

If you're interested in the patent-pending AVAA technology, call up Warren Dent, owner of ZenPro Audio www.zenproaudio.com, the exclusive U.S. distributor of PSI Audio. Warren is a no B.S. guy, and he'll happily let you know how many unicorns were milked to fill the AVAA C20 with magic.

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